AN966 Datasheet, PDF (6/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966 APPLICATION NOTE
Figure 6. Multiplier Block.
Rs
2
3
4
-
E/A
X
+^CURR.CMP
1.7V
D97IN675
CURRENT COMPARATOR AND PWM LATCH (see fig.7):
The current comparator senses the voltage across the current sense resistor (Rs) and, by comparing it
with the programming signal delivered by the multiplier, determines the exact time when the external
MOSFET is to be switched off. The PWM latch avoids spurious switchings of the MOSFET which might
result from the noise generated.
The output of the multiplier is internally clamped to 1.7V, (typ.) thus current limiting occurs if the voltage
across Rs reaches this value.
Figure 7. Current Comparator and PWM latch
Q
4
RS
X
D97IN676
-
+^CMP
1.7V
R
Q
S
DRIVER
DRIVER (see fig.8)
A totem pole buffer, with 400mA source and sink capability, allows driving an external MOSFET. An in-
ternal pull-down circuit holds the output low when the device is in UVLO conditions, to ensure that the
external MOSFET cannot be turned on accidentally.
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